The present invention relates to internal combustion engines and, in particular, to crankcase scavenging two stroke internal combustion engines.
Whilst the invention will be described in relation to gasoline engines operating on a liquid fuel comprising a mixture of gasoline and oil, the invention is equally applicable to diesel engines operating on diesel fuel (liquid) or engines which operate on natural gas or other gaseous fuels.
The term xe2x80x9cfluidxe2x80x9d, as used herein is intended to embrace both liquids and gases and atomized liquids.
The term xe2x80x9cgaseous fuel mixturexe2x80x9d refers to liquid fuel that has been atomised and mixed with air into a gaseous state, or to a mixture of gaseous fuel (eg natural gas) and air.
The term xe2x80x9cfuel free airxe2x80x9d refers to air which has been introduced into the engine without atomising of liquid fuel. The air is therefore substantially fuel free when residual in the transfer passage and when transferred into the cylinder.
The term xe2x80x9ccombustion chamberxe2x80x9d is the zone within the cylinder where the initial combustion of the combustion gases occurs.
The term xe2x80x9ccylinderxe2x80x9d includes the combustion chamber within the cylinder.
The term xe2x80x9cswept volume of the cylinderxe2x80x9d is the volume which is calculated by the piston travel distance, from top dead centre to bottom dead centre, multiplied by the effective internal diameter of the cylinder.
Two stroke engines have been known for many years and have many advantages including their simplicity and ability to be made in small sizes and light weights. In particular two stroke engines find application in many appliances such as outboard motors, mopeds, motor scooters, brush cutters, chain saws, lawn mowers and the like and numbers in use worldwide are in the many tens of millions. A particular disadvantage of prior art two stroke engines is that the scavenging of the cylinder so as to remove the combustion products is accomplished by means of the incoming gaseous fuel mixture. Thus, some of the incoming gaseous fuel mixture passes directly through the cylinder and into the exhaust without having been burnt in the cylinder. As a consequence, there is an inherent loss of efficiency in that a proportion of the fuel consumed is wasted. There is also an undesirable contribution to pollution in that unburnt fuel is allowed to escape into the exhaust system.
In the past there have been various attempts to overcome this higher fuel consumption and in recent times also the high exhaust emission problems. However these various attempts have been operationally complicated, expensive to manufacture, and generally not commercially successful.
Another disadvantage of the conventional two-stroke engine is the relatively high amount of oil that is required to be mixed with the gasoline in order for the engine to be lubricated. This leads to not only higher exhaust emission problems but also higher economic cost in terms of oil consumed.
The following prior art specifications disclosed by novelty searches conducted after the due date, are representation of the prior art.
Japanese Patent 11-82081 (Hirano) discloses an engine of complicated construction. One version has 3 rotary valve mechanisms to control exhaust, air and gaseous fuel mixture respectively. The other versions have a single rotary valve mechanism. These rotary valve mechanisms complicate the construction and cost of such an engine.
U.S. Pat. No. 4,026,254 Ehrlich, U.S. Pat. No. 4,051,820 Boyesen and U.S. Pat. No. 4,067,302 Ehrlich all show inlet valves into the transfer passage without flow control for engine speed. In addition in U.S. Pat. No. 4,067,302 Ehrlich states xe2x80x9cthe air employed to scavenge the cylinder 215 can also be used to ignite unburned combustibles in the exhaust system and thereby to produce an engine with cleaner emissionxe2x80x9d. This clearly demonstrates that gaseous fuel mixture is also used in the cylinder scavenging process.
U.S. Pat. No. 4,481,911 (Sheaffer et al.) discloses a two stroke engine which has the detriment of having gaseous fuel residual in the transfer passage 11 from the previous cycle. Even with air inducted into the central transfer passage 11 as shown at 25, residual gaseous fuel remains at the transfer port end of the transfer passage 11. Consequently, this engine does not have air scavenging of the transfer passage 11 and transfer port 12. Instead gaseous fuel mixture is residual in the transfer passage for the initial scavenging of the cylinder, this gaseous fuel mixture then passing through the exhaust port 16 and into the exhaust passage.
Japanese Patent 02125966-A (Komatsu Zenoah) discloses in FIGS. 1 and 2 an engine which has the problem of residual gaseous fuel mixture in scavenging passage 9 from the previous cycle which is not scavenged at the transfer port end of the scavenging passage 9 by the incoming air. This has the result of gaseous fuel mixture initially scavenging the cylinder and flowing through the exhaust port and into the exhaust passage 13 thus producing additional exhaust emissions. Similarly, the arrangement of FIG. 3 has the problem of residual gaseous fuel mixture in the scavenging passage 9, and in the transfer port 9a and in the air supply passage 10 and 10a. When the piston 4 compresses the gaseous fuel mixture in the crankcase 5, it also compresses the air in the scavenging passage 9 and in the air passage 10. At the same time the crankcase pressure reverses the air flow, thereby causing backwards flow through the venturi of the carburettor 12. This reverse air flow draws extra fuel into the air stream as a gaseous fuel mixture and up into a portion of air supply tube 10. Because of the compression of the gaseous fuel mixture, the residual air in the scavenging passage 9 is forced into air passage 10 by gaseous fuel mixture from the crankcase 5. Thus the scavenging passage 9 is filled with a considerable quantity of gaseous fuel mixture. Hence when the piston 4 moves further towards bottom dead centre and the cylinder scavenging transfer port 9a is opened, the residual gaseous fuel mixture in the scavenge port 9 initially scavenges the cylinder and also flows into the exhaust port 13, thus producing additional exhaust emissions.
U.S. Pat. No. 4,075,985 (Iwai) discloses arrangements in FIGS. 1, 2, 3 and 8 that have the problem of residual gaseous fuel mixture, from the previous cycle, remaining in the corners at the combustion chamber end of the scavenging passages 16 and 25 and of the scavenging (transfer) ports 15 and 24 which are not scavenged by the air. Thus the initial cylinder scavenging is with residual gaseous fuel mixture. The arrangement of FIGS. 1 and 8 also suffer from insufficient volume of the scavenging passages. Iwai""s FIG. 6 shows the scavenging passages 16 connected to air branch passages 17A with reed valves 18 at the start of the air passages. The air branch passage 17A together with scavenging passages 16 form a larger volume but this larger volume in this configuration is detrimental as just prior to the opening of the transfer port 15 with the piston""s downward thrust, the residual air in both the scavenging passage 16 and the air passage 17A is compressed back into the air passage leaving little or no air in the scavenging passage 16 for initial cylinder scavenging.
U.S. Pat. No. 4,253,433 (Blair) shows an extended transfer duct K into which the carburettor fuel is inducted through admission port F and check valve C.
Either fuel free air, or oiled air, is inducted into the crankcase through aperture G and check valve D. If the crankcase air is oiled, then this oiled air, and some fuelled air (as per line 33 and 34 of column 2) is used for initial cylinder scavenging which then proceeds unburnt into the exhaust muffler to add to the exhaust emissions. If the crankcase air is not oiled, then the engine will not function, as the piston and cylinder walls not being lubricated with sufficient gaseous fuel mixture will cause the engine to seize.
U.S. Pat. No. 4,708,100 (Luo) also discloses an engine which suffers from the problem of not being able to function due to a dry crankcase, cylinder and piston and subsequent seizing of the engine due to lack of internal (crankcase) lubrication as only fuel-free (and oil-free) air enters the crankcase.
U.S. Pat. No. 4,948,279 (Luo) also shows a two stroke engine with a complicated and costly rotary valve system which is rotated by gears, belts or chains, and uses compressed air provided by an external source (not shown).
PCT WO 00/43660 (Andersson et al) shows a cylinder (only) with a fresh air port 14 that requires external to the two stroke engine function an air compressing means to enable fresh air port 14 to function. This air compressing requirement means it does not function in synchronism with the pressure changes within the engine.
European Patent 0115758 (Rabl) also discloses a complicated engine which is costly to manufacture and provides insufficient cylinder scavenging by fuel free air to remove the combustion gases from the cylinder. It also suffers from the disadvantage that the air, when it is squirted through the small piston port into the underside of the piston, mixes with some of the gaseous fuel mixture from the remainder of the crankcase.
All of the above mentioned prior art two stoke engines (and cylinders) have other disadvantages in addition to those mentioned above. These are firstly, insufficient volume in their transfer passages to retain residual air for the next cylinder scavenging cycle, and insignificant retention of air, if any, in the cylinder when their exhaust ports are closed.
Secondly, they all have their combustion chambers in the centre or near centre of the cylinder. Thus, on cylinder compression the cylinder contents will not compress rotatingly and turbulently with any possible retained air to uniformly mix with the gaseous fuel mixture in the combustion chamber zone. This shows that they do not retain any significant amount of fuel free air in the cylinder after the exhaust port is closed.
Also all of the above mentioned prior art two-stroke engines (and cylinders) have the additional drawback of requiring the regular quantity of oil in the oil/gasoline fuel mixture, this regular quantity of oil being burnt and exhausted into the atmosphere thus adding to the exhaust emissions.
In many jurisdictions there has been, or will be, an increase in standards in relation to pollution which must be met by two stroke engines. In the United States the Environmental Protection Authority (USAEPA) and in California the Air Resources Board (CARB) are increasingly requiring more stringent standards as time goes by.
The object of the present invention is to provide a crankcase scavenging two stroke internal combustion engine, and a method of operating same, which retains the inherent low cost construction, simplicity and function of a conventional two stroke engine, but which enables the cylinder to be scavenged by fuel free air thereby substantially reducing the amount of any fuel which finds its way unburnt into the exhaust and also reducing the oil requirement for the oil/gasoline fuel mixture, and hence reducing oil consumption.
In accordance to the first aspect of the present invention there is disclosed a crankcase scavenging two stoke internal combustion engine comprising:
an exhaust port openable and closable by a piston reciprocally mounted in a cylinder, said piston being operable to alternatively pressurise and depressurise a crankcase relative to atmospheric pressure, a transfer passage interconnecting said crankcase and cylinder and having a transfer port opening into said cylinder, said transfer port being both openable and closable by the reciprocal movement of said piston, said transfer port being openable after opening of said exhaust port and being closable before closing of said exhaust port; a fuel means to supply fluid fuel into said crankcase and communicating with said crankcase, an air inlet having a unidirectional air inlet valve connected to said transfer passage, said valve being closely adjacent said transfer port, and said air inlet being provided with an adjustable flow controller to adjust the magnitude of air flow through said air supply inlet and thereby adjust engine speed, wherein said transfer passage has a volume which is a substantial fraction of the swept volume of said cylinder, and said air inlet valve is orientated to open towards said transfer port to thereby sweep substantially fuel free air into and past said transfer port and continue into said transfer passage when said crankcase is depressurized, and close said air inlet valve when the crankcase pressure substantially equals or exceeds atmospheric pressure; whereby the substantially fuel free air swept into said transfer passage towards said transfer port blows out of said transfer port and passage the residual gaseous fuel mixture remaining therein from the previous cycle and substantially fills said transfer port and passage with substantially fuel free air which subsequentially is admitted into said cylinder to scavenge same when said transfer port is opened following opening of said exhaust port, some of said air, after scavenging said cylinder, flowing into said exhaust port, and the remainder of said scavenging air remaining in said cylinder following closure of said exhaust port.
In accordance with a second aspect of the present invention there is disclosed a method of operating the crankcase scavenging two stroke internal combustion engine comprising the steps of:
moving said piston to close said transfer port; continuing said piston movement to close said exhaust port to thereby compress the contents of said cylinder and to simultaneously depressurise said crankcase; permitting said air inlet valve to open and introducing substantially fuel free air into said transfer passage towards said transfer port, to blow out of said transfer port and passage any residual fuel/air mixture from a previous cycle; igniting said compressed contents of said cylinder and reversing the movement of said piston; continuing the movement of said piston to close said air inlet valve and pressurise said crankcase; continuing the movement of said piston to open said exhaust port; continuing the movement of said piston to open said transfer port to thereby permit substantially fuel-free air therein to enter and scavenge said cylinder; continuing movement of said piston so that some of said substantially fuel free air enters said exhaust port; continuing the movement of said piston to introduce at least some of the contents of said crankcase into said cylinder via said transfer passage and port to thereby charge said cylinder; continuing said movement of said piston to close said transfer port and said exhaust port; continuing the movement of said piston to mix said substantially fuel free air remaining in said cylinder with said charged cylinder contents; igniting the contents of said cylinder; and repeating the above steps in sequence whilst introducing fluid fuel into said crankcase.
In accordance with a third aspect of the present invention there is disclosed a method of operating a two stroke internal combustion engine to reduce the oil consumption thereof, said method comprising the steps of:
locating a transfer passage between the engine crankcase and engine transfer port, the volume of said transfer passage; being an appreciable fraction of the volume of the swept volume of the cylinder of said engine, after said transfer port closes, scavenging said transfer port and said transfer passage with substantially fuel free air, after said transfer port opens, scavenging said combustion chamber with said substantially fuel free air from said transfer passage, retaining some of said substantially fuel free combustion chamber scavenging air in said combustion chamber to contribute to the combustion gases, and reducing the oil content of the fuel/oil mixture introduced into said crankcase.
In accordance with a fourth aspect of the present invention there is disclosed a method of increasing the richness of the gaseous fuel mixture of a two stroke internal combustion engine at starting, said method comprising the steps of:
locating a decompression valve communicating with the cylinder of said engine adjacent the exhaust port thereof, locating a transfer passage between the cylinder crankcase and engine transfer port, the volume of said transfer passage being an appreciable fraction of the swept volume of said cylinder, after said transfer port closes, scavenging said transfer port and said transfer passage with substantially fuel free air, after said transfer port opens, scavenging said cylinder with said substantially fuel-free air from said transfer passage, closing said exhaust port and charging said cylinder via said transfer port with a gaseous fuel mixture from said crankcase, and maintaining said decompression valve open whilst compressing the contents of said cylinder to expel therefrom some of said substantially fuel free scavenging air otherwise remaining therein, to thereby deplete the air content of said cylinder and is increase the richness of the compressed fuel mixture.